1. Field of the Invention
The present invention relates to an ink set for inkjet recording and an inkjet recording method.
2. Description of the Related Art
Inkjet systems, in which ink is discharged as droplets from an ink discharge orifice, are employed in many printers for reasons such as small size, low cost, and an ability to form an image without contacting a recording medium. Among these inkjet systems, a piezo inkjet system, in which ink is discharged by utilizing deformation of a piezoelectric element, and a thermal inkjet system, in which droplets of ink are discharged by utilizing the phenomenon of boiling of the ink by means of thermal energy, are characterized by their high resolution and high speed printing properties.
At present, when printing by means of an inkjet printer, which involves ink droplets being fired onto plain paper or a non-absorbing recording medium such as a plastic, high speed, high image quality, and good fixation to the recording medium are important issues.
The inkjet recording method forms lines or images from ink droplets by discharging the droplets according to image data onto a recording medium; in particular, if, when recording is carried out on the non-absorbing recording medium, it takes a long time for drying of the fired droplets or penetration thereof into the recording medium, there are the practical problems that the image easily spreads, and adjacent ink droplets on the recording medium mix together, thereby preventing a sharp image from being formed. When there is mixing between liquid droplets, liquid droplets fired next to each other are combined, move, and are displaced from the positions they landed, thus causing nonuniformity in line width when drawing fine lines and causing uneven color, etc. when drawing a colored surface (hereinafter, called ‘fired droplet interference’). Furthermore, there is the problem that, since the degrees to which nonuniformity in line width and uneven color on a colored surface occur vary depending on the ink absorbability and wettability of the recording medium surface, even if the ink used and its discharge conditions are the same, images vary between various types of recording medium.
Moreover, in addition to the above-mentioned problems, an image recorded on a non-absorbing recording medium has problems in terms of image fixation, such as it being easily peeled off and having poor scratch resistance.
As a method for solving the above-mentioned problems, various techniques have so far been proposed.
As an example thereof, in order to impart high definition printing performance, two reactive liquid inks are used, and they are reacted on a recording medium; for example, there have been disclosed a method in which, after a liquid containing a basic polymer is applied, recording is carried out using an ink containing an anionic dye (ref. e.g. JP-A-63-60783 (JP-A denotes a Japanese unexamined patent application publication)), and a method in which, after a liquid composition containing a cationic substance is applied, an ink containing an anionic compound and a coloring material is applied (ref. e.g. JP-A-8-174997).
Furthermore, there has been proposed an inkjet recording method in which a UV-curing ink is used as an ink, dots of the UV-curing ink discharged onto a recording medium are irradiated with ultraviolet rays that match the discharge timing, thus pre-curing them to such a degree that the viscosity increases and adjacent dots do not mix together, and they are then further irradiated with ultraviolet rays and fully cured (ref. e.g. JP-A-2004-42548).
Moreover, there has been proposed a technique in which, after a radiation curing type white ink is applied uniformly on a transparent or semi-transparent non-absorbing recording medium as an undercoat layer and solidified or made to have an increased viscosity by exposure to radiation, inkjet recording is carried out using a radiation curing type color ink set, thus improving the color ink visibility, spreading, and the problem that images vary between various recording media (ref. e.g. JP-A-2003-145745 and JP-A-2004-42525). Furthermore, there has also been proposed a technique in which, instead of the radiation curing type white ink above, a substantially transparent actinic radiation curing type ink is applied by an inkjet head (ref. e.g. JP-A-2005-96254, JP-A-2006-137185, and JP-A-2006-137183).